On the Design Requirement Analysis and RF Validation of High-Rejection Passive Block Bandpass Filter for an Advanced STB Up-Converter

Transcription

1 International Journal of Electronics Engineering, 4 (1), 2012, pp Serials Publications, ISSN : On the Design Requirement Analysis and RF Validation of High-Rejection Passive Block Bandpass Filter for an Advanced STB Up-Converter Rabindra K. Mishra 1, Ajit K. Panda 2 and S.K. Patro 3 1 Department of Electronics Science, Berhampur University, India, prof.r.k.mishra@gmail.com, 2 Department of Electronics and Communication Engineering, NIST, Berhampur, India, akpanda@nist.edu 3 patro_sk@rediffmail.com Abstract: This paper proposes the design requirements and target specifications of a high-rejection radio frequency (RF) Surface Acoustic Wave (SAW) filter for an advanced digital set top box (STB) converter. The target application is in the Cable TV (CATV) domain for both retail customers and high-end cable operators. The RF frequency range of the STB is between MHz for the downstream path and 5-42MHz for the upstream path. The set top box supports both analog and digital video, audio and features. The design analysis and requirements of the SAW type passive bandpass filter has been explained with system level simulations. Simulations have also been carried out for multiple filter combinations to see the overall effect in the block up-converter RF performance. Actual performance validation for been carried out for the RF parameters to support the design analysis. An out of band rejection better than 35dB has been obtained. Keywords: SAW, STB, Block Up Converter. 1. INTRODUCTION Radio Frequency (RF) blocks and sub-systems are an integral part of modern wired and wireless devices and appliances. These sections often perform various analog and high frequency signal processing such as: frequency conversion, signal filtering, band selection, and other important functions. Also, these RF components can be categorized as active and passive devices. From among these devices, Filters are the key blocks to select a desired channel and reject other adjacent and far-apart channels for a noise free operation of the product. Surface Acoustic Filter (SAW) devices are such components which are very critical to the overall operation of many types of consumer products [1-3]. There are different types of SAW devices which are part of the RF sub-sections based on the end product requirement such as programmable type or fixed frequency passive type [4-6]. However, the key benefit of using modern type of SAW filters are: small size, high out-of-band rejection. When it comes to different types of RF system architectures, SAW filtering devices also offer various options to the designer to make a trade-off between a more complex versus a simple and miniaturized architecture. One such use case scenario is in the upconverter or downconverter design where there might be a need for choosing between a simple superheterodyne architecture and a image reject mixer architecture for knocking-off the image frequencies [7]. However, a well-designed passive type high-rejection SAW filter is better than a more complex and cumbersome image reject mixer architecture. This is particularly applicable for a cable television (CATV) STB domain upconverter circuit, where there is a requirement for multiple such types of highrejection filters. A generic CATV STB architecture is as shown in figure 1. Figure 1: Generic Architecture of Modern CATV STB Our present work illustrates the circuit level design requirements for a high-rejection SAW filter as an integral block of the block upconverter (BUC). Simulation is performed for an individual filter section and also for a combination of two such designed filters. Subsequently, RF level test bench validation is carried out for the filters across temperature, as illustrated part of this paper.

2 118 International Journal of Electronics Engineering 2. KEY SAW FILTER DESIGN REQUIREMENTS FOR CATV STB The BUC filters with specific bands are designed for an advanced CATV STB s. The individual passband of these filters are as per the frequency plan mentioned in our paper [8] and listed in table 1. The key specification parameters are insertion loss, out of band and image rejection, group delay performance, in-band return loss, etc. Following sections illustrate in more detail some of these circuit level design requirements. 2.1 Image Filter Positioning in the BUC Section Figure 2 shows the block schematic representation of RF circuit elements for one of the BUC paths. As seen here, single-conversion frequency mixing architecture is used to meet the desired RF performance and also to achieve the small-size and low cost target requirement. Here, an important parameter is the single-ended 50- ohm matched type of requirement which is achieved by using some of the impedance matching techniques [11-12]. 3. CIRCUIT LEVEL LINEAR SIMULATION The circuit level linear s-parameter simulation have been performed for the filter to analyze some of the key parameters such as pass-band insertion loss, return loss, group delay and stop-band rejection. While there are multiple SAW filters with different pass band, we show only the simulation of one of the bands. Other bands are still important and their performance is shown at the validation level. 3.1 Single SAW Filter Simulation The SAW filter.s2p file is used to simulate the bandpass filter of block up converter section. The band3 ( MHz band) bandpass filter in BUC is used in the schematic window. The band3 bandpass filter s2p file has received from the filter vendor and used here for simulation. A sample arrangement of the simulation schematic is as shown in figure 3. The terminating impedance of 50-ohm is used for input and output load. Figure 2: Cascade Arrangement of Single-Block Upconverter of CATV Digital STB Desired noise performance is also achieved with the help of the low noise amplifier blocks placed immediately after the low-pass filtering (LPF) block. The LPF is a very low loss element as used in this scenario. 2.2 Specification Requirements The circuit level design requirements for the BUC section is derived from the system parameters [8] for the STB design. SAW type filters have different RF parameters which are very important to be specified in a proper manner [9-10]. For a high rejection requirement, Table 1 lists these parameters for individual SAW type bandpass BUC filters with high out-of-band rejection. Table 1 SAW Filter Specification Requirements RF Parameters Specification Units Centre Frequency 81, 159, 255, 351, MHz (for individual BUC filters) 447, 525 In-band Insertion Loss (max.) 23 db Amplitude Variation 3 db Out-of band attenuation 10 db (at 6MHz away from band-edges) Image frequency 35 db rejection (typical) Source/Load Termination 50 Ohm (single-ended) Operating Temperature 20 to +70 deg. C Figure 3: Single BUC Filter Simulation Schematic As seen from figure 4, input Return loss is better than 3.9 db and output return loss is better than 3.8dB in the passband of 252MHz to 342MHz. While the in-band insertion loss is 16.4dB at 252MHz, a loss of 16.1dB is achieved at 342 MHz. Figure 4: BUC Filter Simulation Performance for Band-3 Out of band atten uation (in cluding th e image frequencies) of better than 60dB is obtained from 90MHz to 213MHz and 377MHz to 500MHz. The obtained value of the group delay is 282nS. 3.2 Dual SAW Filter Simulation Here, two bandpass SAW filters (band-3, 4) are passive combined to replicate the scenario of block up converter section and simulation have been performed. The combiner used is an ideal element with minimum loss.

3 On the Design Requirement Analysis and RF Validation of High-Rejection Passive Block Bandpass Filter Figure 5: Passive-combined Simulation Schematic of Two BUC Filters As noticed from figure 6, passband insertion loss is 19.4dB and 19.1dB for 252MHz and 342MHz respectively while it is 21.5dB from 348MHz to 538MHz. Combining loss of 3dB can be attributed to this additional loss. Higher side falling edge of band3 SAW is 19.7dB at 344MHz while lower side falling edge of band3 SAW is 24.7dB at 344MHz. Also, another plot is shown here which depicts the close-in view of the intersection points of the two filters. 4. TEST SETUP AND MEASURED RESULTS A basic test setup is shown in figure 8 for evaluation of the SAW filter sections, alongwith th e environmen tal temperature chamber. The Vector Network Analyzer setup is calibrated for a full 2-port s-parameter for both magnitude and delay performance prior to connecting the Device Under Test (DUT). One-side open-ended 50-ohm RF cables are connected to the input and output of the DUT and in-turn to the VNA ports. The frequency is swept for each individual BUC paths and measurements are performed. Figure 8: Test Setup for the SAW Filter Section Evaluation Some of the other instrument settings of the VNA are done as per the following: IF Bandwidth-70kHz, RF Power: -10dBm, Slope: 0 db/ghz. Figure 6: Combined BUC Filter Simulation Performance for Band-3, 4. As seen from the above results, one of the filters has about 2dB higher insertion loss as compared to the other one. This unequal loss is compensated with the help of pi-type matching-pad in the actual BUC section. Also, as we see in figure 4, the return loss is poor for the SAW filter for the inband frequencies and this shall be further improved in the actual circuit implementation with the help of matching pad sections for individual BUC paths. Further, the multi-way passive combined performance for all of the SAW filter paths have been simulated and are shown in figure 7, alongwith the close-in view of two consecutive bands. Figure 7: Combined BUC Filter Simulation Performance for all the Bands It is seen here that with the proper equalization of the individual path losses, overall in-band flatness is achieved. This is very critical for meeting the RF signal level at the STB output. Figure 9: Environmental Testing at 25 C (Ambient-Room Temperature)

4 120 International Journal of Electronics Engineering Figure 10: Environmental Testing at 70 C (Hot Temperature) Figure 11: Environmental Testing at 25 C (Cold Temperature) As seen from above figures (figure 9 till figure 11), the in-band insertion loss is 21dB or better for the first SAW filter at all temperatures and this is in agreement with the desired specification requirement. However, the second SAW filter is having 3dB more insertion loss in the midband and high-band frequencies. While the additional loss can be compensated with the help of the matching pad, the in-band slope can be flattened with the help of passive-type equalizer circuits having reactive circuit elements. The inband poor return loss is a acceptable for a high reject SAW filters and this can be improved further with the help of the above mentioned matching pad elements. For all the temperature points, the group delay flatness is achieved in the passband as noticed here. 5. CONCLUSION In the present work, high-rejection SAW filter specification is analyzed in terms of the circuit level simulations and followed by actual design validation over three temperatures. The simulation and test results are in close agreement with each other. The suggested types of filters are found to be very useful for the STB from system requirement point of view, to achieve the desired RF performance. Similar type of filter topology can be used for related applications which require block frequency conversion while maintaining a compact overall system dimension. References [1] Colin K. Campbell, Surface Acoustic Wave Devices for Mobile and Wireless Communications, Volume 1-4 of Applications of Modern Acoustics, Academic Press: Boston, 633 pages, [2] Clemens C.W. Ruppel, et.al., SAW Devices for Consumer Communications Applications, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, pp , 40(5), September, [3] SAW Devices for GPS, RKE, RFID, TTI RF Product Webinar Presentation Material, Murata Electronics. [4] T. Kenny, et. al., Wideband Programmable SAW Filters, pp , IEEE Ultrasonics Symposium, [5] SAW Filter Products, Vectron International Product Brochure. [6] SAW Filter for Up-Down Converters Used in TV Silicon Tuners, Fujitsu Technical Feature, pp. 1-3, 26, 2008.

5 On the Design Requirement Analysis and RF Validation of High-Rejection Passive Block Bandpass Filter [7] Cotter W Sayre, Complete Wireless Design, Book Chapter 7 (Mixer Design), pp , Mc Graw Hill Publishing, June, [8] Rabindra K. Mishra, Ajit K. Panda, Saroj K Patro, A Suitable Upconverter Architecture and Design Considerations for Digital Set Top Box for CATV Applications, IEEE Applied Electromagnetic Conference, Kolkata, India, December [9] How to Specify a Custom SAW Filter, COM DEV SAW Products Application Note 103. [10] D. Adams, The Effects of SAW Group Delay Ripple on GPS and Glonass Signals, Technical Paper. [11] Impedance Matching of SAW Filters, COM DEV SAW Products Application Note AN001. [12] Optimize the MAX2338 Mixer IF SAW Filter Match at 183.6MHz for CDMA Application, Maxim Application Note 916, May 2002.